1. Field of the Invention
The present invention pertains to a system for the display of data, represented in the form of a luminous image, the readability of which is improved in order to take into account fluctuations in the light environment.
Luminous data can be represented as either electronic images or luminous displays.
The invention is envisaged, more particularly, for head-down display devices in aeronautics.
2. Description of the Prior Art
The display of an electronic image, especially for a head-down application, is generally done on the screen of a cathode ray tube or on a flat panel. There are two main categories of flat panels, passive display flat panels which modulate light, for example, with liquid crystal cells, and active display flat panels that emit light, for example, plasma panels or electroluminescent panels.
In these systems, the luminous display is subjected to illumination which varies because of the light environment which itself varies. This can cause difficulties related to the readability of the display. This is especially so when the local illumination is too great to display an image on a cathode screen.
There are known methods for manually adjusting the luminance level of an image in electronic image display devices.
Such adjustment is inadequate because, in the case of head-down display on an instrument panel, the electronic image is shown to the pilot in a field of about 20.degree. and the total image, seen by the observer, consists of this electronic image on which an unwanted light distribution is superimposed owing to the environment. In the 20.degree. field of vision, the unwanted light distribution is the ambient radiation reflected by the display screen. Furthermore, the unwanted light radiation comprises the radiation included in the observer's total field of perception, the value of which is practically 180.degree.. For a head-down application envisaged, this unwanted light is the light coming from outside the aircraft through the canopy of the cockpit.
An appreciation of the quality of the total image depends on the size of this unwanted luminance. The quality of the image further depends on the spatial/temporal distribution of the entire field perceived by the eye, i.e. firstly, on the value and spatial distribution of the light environment including the displayed image and the reflected unwanted radiation and the rest of the perceived visual field and, secondly, the value and spatial distribution of this light environment at prior moments, so as to take into account, in particular, retinal persistence on the part of the observer and slow variations in the diameter of the pupil.
An aim of the invention is to improve the readability of luminous displays taking into account this spatial/temporal distribution of luminance, corresponding to the unwanted light reflected by the display screen and the unwanted light perceived in the total field outside the display vision field.
According to the invention, the display system has sensor devices to measure this unwanted radiation by photodetection and to process it so as to affect the luminance of the displayed image in order to optimize its readability.